The study of specific enzyme mutations using electrophoresis has the capability of detecting mutations that result in either a non- functional protein or a functional protein with altered electrical charge. Our objective is to develop a single mammalian test system that will detect both electrophoretic mobility (EPM) and inactive enzyme (IAE) mutations in the same loci. Two inbred mouse strains which differ with respect to nine enzyme loci have been selected for this system. Adult males of each strain were treated with gamma radiation and mated to unirradiated females of the other strain to produce F1 hybrid offspring. Tissue samples are obtained surgically and electrophoresis is conducted on crude homogenates. While sampling is not yet completed, preliminary data indicate that the system is at least as sensitive in detecting IAE mutations as is the morphological specific locus system. Several mutants have been obtained and currently studies are underway to determine the histochemical nature of the induced mutations.